Kernel of a directed graph
= Kernel of a directed graph
= Digraph kernel
{synonym}
A digraph kernel is an <independent set> $K$ such that each <vertex> outside $K$ has a <directed edge> to a vertex in $K$. A directed cycle of even length has a kernel, obtained by taking alternate vertices; a directed triangle has none. The <kernel-perfect directed graph> property requires existence of such a set in every <induced subgraph>.